The field of the present invention is ultrasonic medical diagnostic equipment for non-destructive testing and non-invasive examination of soft-tissue and body organs and specifically equipment for testing the accuracy of and calibrating ultrasonic diagnostic equipment such as pulse echo body scanners and the like.
Apparatus and techniques which permit the non-destructive testing and non-invasive examination of soft tissue and body organs are of particular interest to the medical community. Examples of presently available techniques include x-ray, nuclear medicine, thermography and diagnostic ultrasound. Ultrasonic diagnostic techniques are important because they offer a very high benefit to risk ratio for the patient and the ability to perform quality imaging of soft tissue organs. Ultrasonic diagnosis has found widespread applicability to the medical subfields of obstetrics, gynecology, cardiology, neurology, opthalmology, and urology, among others. Ultrasonic diagnostic has proved of particular value as a diagnostic aid for the pregnant uterus including fetus and placenta, eye, breast, brain, lung, kidney, liver, gall bladder, bile ducts, pancreas, spleen, heart and blood vessels and soft tissues of the neck, including thyroid and parathyroid glands.
Ultrasonic diagnostic instruments operate on either a pulse-echo or Doppler principle. These principles are both well known. Most frequently the imaging of soft body tissue is accomplished using the pulse-echo principle. Short bursts of ultrasonic energy are transmitted into the body and the echoes are recorded. The time required for an emitted pulse to return as an echo provides an indication of the distance of a measured structure. Echoes occur at the boundaries between different tissues within the body since a fraction of the incident energy is deflected whenever the characteristic impedance of the structure under examination changes. Typically a change in the characteristic impedance occurs at such a boundary. Impedance is defined as the product of the density of the tissue multiplied by the velocity of sound. The first boundary will not typically reflect all the energy which may be reflected at subsequent boundaries. Thus, various boundaries at various depths can be observed.
Ultrasonic diagnostic equipment is used by a process called scanning. Scanning involves the movement of a pulsed sound beam promulgated by a transducer through a scanning plane. The transducer converts electrical signals into acoustic pulses and converts the returning echoes back into electrical signals. Through scanning, a two dimensional image of the various organs or body regions of interest can be generated.
The quality of the two-dimensional image generated through the scanning process is dependent on the axial, lateral and elevational (i.e., out-of-plane) resolution of the transmitted ultrasonic beam and the absence or presence of side lobes. Resolution is also substantially dependent on the cross-section of the ultrasonic beam of various depths. Various methods and devices have been proposed for determining axial and lateral resolution of the ultrasonic beam and the beam within the direction of the scanning plane. In a copending application, Ser. No. 097,599 now U.S. Pat. No. 4,903,523, an apparatus and method are disclosed for measuring the out-of-plane width (i.e., elevational resolution) of the beam. This measurement is important because ultrasonic testing equipment operates on the fictitious assumption of a strictly one dimensional beam. Because the beam is actually three dimensional, out-of-plane reflections from objects in the scanning volume may be displayed as if they were within the theoretical scanning plane, thus creating undesirable image effects. Copending application Ser. No. 097,599 discloses a test object comprising one or more wedge-shaped clusters of targets whose apices are positioned in the scanning plane. By placing a series of target clusters at different depths, out-of-plane beam width (elevational resolution) can be measured throughout the scanning range.
In order to assure accuracy in imaging out-of-plane beam width, it is necessary to orient the transducer such that the scanning plane passes through the apex or apices of the target wedge(s). If the transducer is allowed to pivot or rotate such that the scanning plane does not pass through the target apex or apices, inaccurate measurements may result. In the present state of the art, the ultrasonic transducer is hand held for positioning the transducer on the scanning surface of the test object or within an open tank of water containing the test object. This procedure requires considerable skill and patience to obtain and maintain control of the optimum position of the transducer in relation to the test object which is essential for accurate measurement of the beam slice thickness. Because the optimum position of the transducer requires that the scan plane coincide with the plane of the wedge apices, misalignment over two degrees of freedom of motion along the vertical and longitudinal axis of the transducer must be avoided. Thus, means for fixing a transducer with respect to a desired scanning plane would be advantageous.